Coated polymeric particles having improved anti-block characteristics, method of making such particles, and apparatus therefor

ABSTRACT

To provide superior anti-agglomeration, or tackiness-reducing, properties, one aspect of the invention is directed to coated polymeric particles, with each of the coated particles comprised of a polymeric substrate particle and a block-reducing coating on the surface of the substrate particle. The coating includes polymeric coating particles which advantageously may be in the form of a micro-fine powder. Also disclosed herein is a process for applying the polymeric coating particles onto the polymeric substrate particles, as well as equipment which is especially useful in producing the coated polymeric particles.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to polymeric particles having a coating whichprovides improved anti-blocking properties, and to a method ofmanufacturing the coated polymeric particles. The invention alsoencompasses equipment used in manufacturing the coated polymericparticles.

A variety of polymeric materials are produced from an initial reactionsequence at elevated temperatures and pressures. The resulting polymericmaterials exist in a molten form. The polymeric materials may be furtherprocessed directly from the molten phase, or be cooled to ambienttemperature prior to further processing. Depending on processingconditions and composition of the reactants, the cooled polymericmaterials may be converted to pellets or other physical forms which maybe easily packaged or may be first cooled and then ground, chopped orotherwise processed prior to further processing of the material. Somepolymeric materials, by the nature of their composition, tend to exhibitcold-flow properties. Examples of such polymeric materials are ethylenevinyl acetate, very low density polyethylene (i.e., polyethylene with adensity of 0.90 grams per cubic centimeter or less), ethylene methylacrylate, and ethylene n-butyl acrylate. These materials, even thoughpreviously processed to produce particulate units, such as pellets,chips or powders, will nonetheless tend to flow at ambient temperaturesand pressures. The particles, after cold-flow, tend to agglomerate toform a single mass which is difficult to reinstitute into the componentpellets, granules or the like. It is highly preferred that polymericmaterials which exhibit these cold-flow tendencies continue to havefree-flowing characteristics.

Various attempts have been made to provide a surface coating topolymeric particles to limit or eliminate the tendency to agglomerate.To this end, materials such as bisoleamide have been incorporated intothe polymer reaction mixture prior to formation of the polymer particle.The bisoleamide is not miscible with the polymeric material and, intime, migrates to the surface of the particle to provide a coating whichresists agglomeration. It has also been known to coat silica and talcpowders onto the surface of a polymer particle to inhibit agglomeration.

It is also known to apply a micro-fine polyolefin powder coating ontopolymeric pellets to decrease the tackiness of the pellets. Themicro-fine powder can be applied by incorporating the powder into thechilled water of an underwater pelletizing device which cools thepellets cut after extrusion. Alternatively, the micro-fine powder can becoated onto formed polymer pellets by tumbling, airveying or the like.Also, the coating may be applied by electrostatically charging a bath offluidized powder with an electrical potential different from that of thepolymer pellets.

In the preparation of polymeric particles having improvedanti-agglomeration properties, there remains a need for a product whichnot only exhibits superior anti-agglomeration properties followingtreatment with a coating material, but also retains this property afterpack(aging, transport, and in further processing.

SUMMARY OF THE INVENTION

To provide superior anti-agglomeration, or tackiness-reducing,properties, one aspect of the invention is directed to coated polymericparticles, with each of the coated particles comprised of a polymericsubstrate particle and a block-reducing coating on the surface of thesubstrate particle. The coating includes polymeric coating particleswhich advantageously may be in the form of a micro-fine powder. Alsodisclosed herein is a process for applying the polymeric coatingparticles onto the polymeric substrate particles, as well as equipmentwhich is especially useful in producing the coated polymeric particles.

The coated polymeric particles may be produced by incorporating thepolymeric coating particles into an aqueous coating composition andapplying the composition to the surface of the substrate particles suchas by spraying. The resulting coated particles then may be subjected toa drying step, such as in a fluidized bed, to remove substantially alltraces of water and other volatile components from the coated particles.

The method of forming the coated particles may include introducingpolymeric substrate particles into an auger assembly which conveys thesubstrate particles through a spray zone to an outlet zone. Duringoperation of the auger assembly, the substrate particles are sprayedwith an aqueous coating composition as the particles are mechanicallymixed while being conveyed forward. From the outlet zone, the resultingcoated particles may be conveyed to a drying apparatus such as afluidized bed. The fluidized bed operates at elevated temperatures andprovides an air flow which thoroughly agitates the coated particles.This combination of elevated temperature and agitating air flow removeswater and other volatile components. The coated particles thereafter arecooled and transferred to a packaging or storage station. Preferably,the coated particles are cooled in a downstream section of the fluidizedbed prior to transfer to the packaging or storage station.

If desired, the auger assembly may include a screw mounted inside achamber, preferably such as a barrel, wherein the outer diameter of thescrew is slightly less than the inside diameter of the barrel to therebyprovide minimal clearance between the crest(s) of the screw helicalflight(s) and the interior wall of the barrel. This arrangementsubstantially prevents backflow of particles toward the inlet of theauger assembly. If desired, the screw diameter and screw pitch may beuniform along the length of the screw, although this is not required.The pitch is the length of a longitudinal section of the shaft of thescrew occupied by a flight, a flight being any full, 360 degree rotationof the spiral-like portion of the screw which extends radially outwardfrom the shaft of the screw. At least one spray head is mounted tointroduce aqueous coating composition into the barrel. In another aspectof the invention, the barrel of the auger is fitted with at least twospray heads for introducing the aqueous coating composition containingthe polymeric coating particles into the barrel and into contact withthe polymeric substrate particles. In an embodiment of the augerassembly where two or more spray heads are employed, the screw has auniform pitch along its length, and the spray heads are positioned in astraight line on the barrel parallel with the longitudinal axis of thescrew and of the barrel, the distance along the longitudinal axis of thebarrel between spray heads is advantageously a non-integral multiple ofthe pitch of the screw. With the spray heads configured in this way, theflow of aqueous coating composition through the spray heads into thebarrel is never completely interrupted during rotation of the screw.

In a further aspect of the invention, the spray heads are mounted sothat the orifice of each spray head nozzle is substantially flush withthe interior cylindrical surface of the barrel. In this fashion, themovement of crests of the screw, the particles, or both, across theflush-mounted spray heads minimizes the formation of coating compositionsolids build-up inside the auger assembly. As a result, auger assemblydowntime is minimized and the substrate particles are more uniformlycoated with the desired concentration of polymeric coating particles.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of this invention are described in more detail in thefollowing description and in the drawings, in which

FIG. 1A is a scanning electron micrograph (SEM) of a coated pellet madein accordance with the principles of the invention;

FIG. 1B is a scanning electron micrograph (SEM) of a portion of thecoated pellet of FIG. 1A;

FIG. 2 is a block diagram of a process for producing coated pellets inaccordance with the principles of the invention;

FIG. 3 is an enlarged, partial cross-sectional view of a particle feederand auger assembly according to the principles of the invention; and

FIG. 4 is an enlarged, partial cross-sectional view of several sprayheads mounted to the auger assembly of FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

One aspect of the invention is directed to a coated polymeric particlehaving improved anti-block characteristics including a polymericsubstrate particle and a block-reducing coating on the surface of thepolymeric substrate particle, the block-reducing coating comprised ofpolymeric coating particles.

The term “particle”, as used herein, refers to the physical form orshape of the polymeric materials of the invention, and includes, forexample, a pellet, granule, chip, powder, flake, sphere, or any otherform or shape suitable for use as either a substrate or a coatingmaterial.

In another aspect of the invention, the substrate particle and coatingparticles are formed of a polymer derived from the same monomer,comonomer or termonomer system to manufacture the ultimate polymer. Thecommon monomer, comonomer or termonomer system is identified herein asthe “chemical family”. Also, as used herein, the term “polymer”encompasses homopolymers, copolymers, and terpolymers. The feature ofproducing a coated particle from substrate and coating particles of thesame chemical family offers several benefits, including, for example, aresulting coated polymeric particle with enhanced predictability ofsubsequent processing and performance characteristics. By way ofnonlimiting example, distinct chemical families, as used in conjunctionwith the invention, include: ethylene vinyl acetate, ethylene vinylacetate polyisobutylene, ethylene maleic anhydride, ethylene methylacrylate, ethylene butyl acrylate, polyethylene, styrene butadiene,silane, ethylene polypropylene diene, polyurethane, polyisobutylene,butyl rubber, and combinations thereof.

The polymeric coating particles generally have an average diametersubstantially smaller than that of the polymeric substrate particle. Forexample, the average diameter of the polymeric coating particlespreferably is 100 microns or less, more preferably 30 microns or less,and most preferably 20 microns or less. In one aspect of the invention,the polymeric coating particles have an average diameter in the range of10 microns to 30 microns. In contrast the polymeric substrate particlehas a generally spherical shape with a diameter in the range of about 90mils to 150 mils, with a preferred particle diameter of 125 mils whenthe substrate particle is in the form of a pellet.

The polymeric coating particles are coated onto the polymeric substrateparticle by application of an aqueous coating composition containing thepolymeric coating particles. The composition advantageously may becomprised of a binder material such as an emulsifiable wax, a base suchas potassium hydroxide, surfactants, an antimicrobial agent, an antifoamagent, polymeric coating particles, and deionized water. One factor indetermining the maximum concentration of polymeric coating particles inthe composition is the viscosity of the composition. More specifically,depending upon how the composition is applied to the substrateparticles, the composition's viscosity should be low enough to allow thecomposition to be pumped through an application system and to flowsatisfactorily over the polymeric substrate particles.

If desired, a carrier system commercially available from the EastmanChemical Company, Kingsport, Tenn. may be used to advantage informulating the aqueous coating composition. The particular carriersystem is sold under the product name Aquastab®, and typically includesthe following components:

TABLE 1 Component % (by weight) Carrier Components 50-75 Binder material3-8 Surfactants 2-4 Potassium hydroxide 0.1-0.2 Antifoam additive0.2-0.4 Antimicrobial agent 0.001-0.2  Deionized water   37-63.2Polymeric additive(s) 25-50

When such an Aquastab® carrier system is employed, the polymeric coatingparticles are combined with the carrier components listed in Table 1 toform the aqueous coating composition. The process of preparing suchcoating compositions, and representative coating compositions, aredescribed in more detail in U.S. Pat. Nos.: 4,880,470; 4,898,616;4,960,644; 4,975,120; 5,007,961; 5,041,251; 5,096,493; 5,190,579;5,334,644; and 5,443,910, the entire disclosure of each of these patentsbeing incorporated herein in its entirety by reference.

One particularly suitable aqueous coating composition includes anethylene-vinyl acetate copolymer (EVA) powder, such as Microthene® FE532-00 commercially available from Equistar Chemicals, LP, Cincinnati,Ohio, dispersed in an aqueous emulsion containing carrier componentssuch as those listed in Table 1. The Microthene FE 532-00 coatingparticles are substantially spherical in shape. The EVA powder comprisesapproximately 40% by weight of the total aqueous coating composition.With respect to physical characteristics, this composition has a whiteto off-white appearance, a pH in the range of 7 to 10, an amount ofwater in the range of 47.1% to 48.1% by weight and an amount ofMicrothene® FE 532-00 in the range of 39.5% to 40.5% by weight. Inaddition, the composition has the following viscosity/dilution curvecharacteristics: where the Microthene® FE 532-00 EVA powder is presentin an amount of 40% by weight, the viscosity, as measured in centipoise(cP) units, is typically 30.6; at a concentration of 35% by weight, theviscosity usually is 14.9; at a concentration of 30% by weight, theviscosity typically is 9.1; and at a concentration of 25% by weight, theviscosity generally is 6.7.

The polymeric material of the substrate particle which has improvedanti-block properties after application of coating particles may have amelt index of at least 15 grams per ten minutes (g/10 min), preferably150 g/10 min, and most preferably 150 to 800 g/10 min. The melt index ismeasured by employing ASTM No. D 1238 which is incorporated herein inits entirety by reference.

By utilizing coating particles having a composition of the same chemicalfamily as the substrate particles, the resulting coated polymericparticles offer an enhanced degree of predictability and purity insubsequent processing. By way of non-limiting example, the above aqueouscoating composition has imparted superior anti-blocking properties toEVA substrate pellets such as Ultrathene®UE 653-04 pellets availablefrom Equistar Chemicals, LP, Cincinnati, Ohio, as described in moredetail below. Though the contact area between pellet and coatingparticle is small relative to the respective diameters of the pellet andcoating particle, an anti-block coating is provided which resistsremoval by vibration, contact with adjacent coated pellets, and contactwith the walls of the particular transfer and storage equipment. Whilebeneficial anti-block results may be achieved with EVA substrateparticles having a range of vinyl acetate concentrations, beneficialresults may be achieved by applying coating particles to EVA substrateparticles having at least about 18% by weight vinyl acetate,advantageously where the substrate particles have at least about 22% byweight vinyl acetate, and more advantageously where the substrateparticles have at least about 25% by weight vinyl acetate. Additionally,increasingly beneficial results may be observed in connection withapplying coating particles to EVA substrate particles having a meltindex of at least 25 grams per ten minutes (g/10 min), at least 100 g/10min, at least 400 g/10 min, and a range of 150 to 800 g/10 min. Thetendency of EVA substrate particles to block increases with increasingmelt index and vinyl acetate content. This increasing blocking tendencywill also be found in substrate particles of other chemical families asthe melt index increases. It is thus expected that greater improvementsin anti-blocking properties will be found by applying the coatingparticles of the invention to substrate particles having a greaterinitial tendency to block.

In addition, when ethylene vinyl acetate polymeric coating particles areused, it is advantageous to employ coating particles having a vinylacetate concentration of at least 4% by weight vinyl acetate, preferablyfrom 4% to 12% by weight. In addition increasingly beneficial resultsmay be realized when the EVA coating particles have a melt index of atleast about 3 grams per ten minutes (g/10 min), preferably about 8 g/10min. Generally, the upper melt index limit for the coating particle isabout 200 g/10 min.

Where the polymeric substrate particle is a pellet, the pellet may beproduced by one of several known pelletization techniques. As describedherein pellets are formed by extruding molten polymeric material throughan extruder head and cutting the individual pellets in a chilled waterbath. The wet pellets are substantially completely dried by removal ofmore than 99% of the water, such as in a spin dryer. The pellets arethen conveyed to the hopper of an auger assembly, with any agglomeratesof the pellets being broken up by an agitator in the hopper. The pelletsare then conveyed through the auger assembly barrel by a rotating screwand subjected to an aqueous coating composition spray from at least onespray head flush mounted on the inside diameter of the barrel. The augerassembly operates at ambient temperature and pressure, and the screwoperates at a speed dependent on the output rate of the polymerizationreaction, but is generally configured to operate in the range of about45 to about 70 revolutions per minute. The agitator operates atapproximately one third the speed of the auger screw, or at about 20revolutions per minute. Optionally, a small quantity of polymericcoating particles is introduced into the underwater pelletizer water,typically about 0.05% by weight of the underwater pelletizer water, topromote flow of the substrate pellets.

The concentration of coating particles on a 100% solids basis istypically from about 2,000 to about 10,000 ppm based on the weight ofpellets processed through the auger assembly. The preferred coating rateis about 4,000 ppm on a 100% solids basis of coating particles. Theaqueous coating composition containing ethylene vinyl acetate coatingpowder and having the proper operating viscosity may contain up to about40% coating particles. Thus, at the preferred coating rate the aqueouscoating composition is applied at a rate of 1% by weight of the pelletsprocessed through the auger assembly. The aqueous coatings compositionmay also be applied after dilution with additional water. The coatingparticle content of the aqueous coating composition generally rangesfrom about 20% to about 40%.

It can be appreciated that the coating particles are advantageouslyapplied to improve the anti-block characteristics of the substrateparticle. The concentration of the coating particles to impart thedesired anti-block characteristics will vary as a function of the sizeand shape of both the substrate and coating particles, the compositionof both the substrate and coating particles, and the tendency of theuncoated substrate particles to block. Thus, actual acceptable coatingparticle concentrations may vary from the above typical range.

In a further aspect of the invention, the aqueous coating compositioncontaining the polymeric coating particles advantageously is applied tothe polymeric substrate particle through at least one spray head whichis flush-mounted on the barrel of an auger assembly into which thepolymeric substrate particles are introduced. Generally, the polymericsubstrate particles are introduced into an auger assembly operating atambient temperature and pressure via a hopper or inlet chamber. However,the auger assembly may alternatively be cooled to further decrease thetendency of the substrate particles to stick to each other or the wallsof the auger assembly. The substrate particles then come into contactwith the aqueous coating composition while being conveyed through achamber, such as a barrel, by the screw. An advantage of the use of ascrew or similar device is that the substrate particles and aqueouscoating composition become more completely mixed as the particles aretransferred along the chamber and contact other particles. Preferably,at least two spray heads are utilized. When oriented in a straight lineparallel with the axis of the barrel, the spray heads are spaced apart adistance which is a non-integral multiple of the pitch of the screwwhere the screw pitch is constant. This spacing ensures that theintroduction of the aqueous coating composition into the interior of theauger barrel will not be completely interrupted at any time, as theindividual crests of the screw pass across the individual spray heads.Alternatively, the spray heads may be positioned so that the heads arenot oriented in a straight line parallel with the barrel's longitudinalaxis. Moreover, the spray heads may be mounted in any orientation aboutthe circumferential sidewall of the barrel. In this orientation, theheads again are preferably positioned so that the flow of aqueouscoating composition is not completely interrupted when the crest of ahelical flight passes across an individual spray head. Also, the crestthickness of individual helical flights of the screw in the vicinity ofa spray head may be narrowed so that interruption of liquid flow throughthe spray head is minimized. It is also contemplated that the screwpitch may vary, and in this embodiment, the spray head(s) would bemounted such that flow of aqueous coating composition into the barrel isnot completely interrupted when the flight crests pass across the sprayhead(s). In addition, though the substrate particles are coated andtransferred preferably along a closed conveyance or chamber having asingle inlet and a single outlet, such as a barrel, any suitable chambermay be used. For example, it is contemplated that the conveyance mayinclude openings in addition to the inlet and outlet, and may forexample have a continuous open upper portion wherein the conveyanceapproximates a trough in appearance. Where the conveyance is open to theenvironment, overspray may occur, and thus additional containmentmeasures may need to be taken.

Where a closed conveyance such as a barrel is used, the flush-mountingof the spray heads at the inside diameter of the barrel and the closespacing between screw crests and inside barrel diameter minimize thebuild-up of aqueous coating composition in the vicinity of the sprayheads. As a result, the coating operation can operate on a substantiallycontinuous basis. If cleaning is required, a water flush is generally asufficient treatment, involving minimum equipment down time.

The diameter of the spray head orifice through which the emulsion isapplied is preferably at least twice the average diameter of thepolymeric coating particles so as to minimize the risk of plugging.

After the substrate particles are coated in the auger assembly, thewater and any residual volatile emulsion or particle components aredriven off in a fluidized bed, which tumbles the coated pellets at anelevated temperature and air flow, followed by passage through a coolingregion in the fluidized bed, prior to outputting the dried coatedpellets to transfer equipment. It is believed that a substantial portionof the non-aqueous emulsion components, other than the coatingparticles, is volatilized and removed from the coated particles. It isbelieved that at least a portion of the bonding needed to maintain thecoating particles on a substrate particle is attributed to theemulsifiable wax component of the emulsion. Where the substrate andcoating particles are produced from a polar polymer, ionic and/orcovalent bonding may also contribute to maintaining the coatingparticles on the particular substrate particle. In addition, weakhydrogen bonding effects may also contribute to maintaining the bond ofcoating particles onto the substrate particle.

In one version of the invention, the coating particles represent atleast 0.3%, at least 0.4%, or at least 0.5% by weight of the coatedpolymeric particle. Though coating particle levels greater than 0.5% byweight may be utilized, the improvement in anti-block propertiesmeasured by stick temperature (discussed in detail below) tends toincrease at a slower, non-correlatable rate.

As indicated above, improvements in anti-block properties are evaluatedby measuring the stick temperature of coated particles. It has beenfound that the coated particles of the invention exhibit at least a 20%improvement in stick temperature compared to the uncoated substrateparticle, preferably a 40% improvement, and most preferably at least a60% improvement.

Referring to FIGS. 1A and 1B, a coated polymeric pellet formed accordingto the principles of the invention is shown. In FIG. 1A, the length ofthe micron bar represents one millimeter, and in FIG. 1B, the length ofthe micron bar represents 100 micrometers. The coated pellet includes anUltrathene®UE 653-04 ethylene-vinyl acetate copolymer (EVA) substratepellet having a block-reducing surface coating, with the surface coatingincluding polymeric coating particles in the form of Microthene® FE532-00 EVA powder, the coating particles representing about 0.40% byweight (i.e., 4000 ppm) of the total weight of the coated pellet shown.This particular substrate pellet has a generally egg-like shape, whilethe block-reducing EVA powder particles are generally spherical.

Referring to FIG. 2, substrate pellets 10 are conveyed to a substratepellet feeder 16 for introduction into an auger assembly 18 which willbe described in more detail below, with reference to FIGS. 3 and 4. Theaqueous coating composition 12 is applied to the surface of the pellets10 in the auger assembly 18. The coated pellets (not shown) are thendried in a fluidized bed, where they are exposed to an elevatedtemperature and a high-velocity air flow, which serve to thoroughlyagitate the coated pellets while water and volatile components areremoved from the coated pellets. The dried, coated pellets are thencooled, preferably in a down-stream section of the fluidized bed 24,prior to transfer to a coated pellet storage/packaging station 26 orother appropriate processing station.

In an alkaline environment, certain antioxidants such as butylatedhydroxytoluene (BHT) will discolor, imparting a typically yellow colorto the coated particle. It has been found that the combination of spraycoating in an auger assembly with fluidized bed drying/agitating, asdescribed in more detail below, tends to minimize formation of colorbodies on the coated particle.

Referring to FIG. 3, several spray heads 30 are mounted on the augerassembly 18, and more specifically, on a chamber referred to as a barrel32, with the spray heads 30 used to spray the aqueous coatingcomposition 12 onto the substrate pellets 10 as they move through theauger assembly 18. The substrate pellets 10 are fed into the particlefeeder 16 through an inlet port 34, where they pass to a feeder section36. The pellets 10 fall by gravity from the feeder section 36 into anagitator chamber 38. An agitator 44 is mounted within the agitatorchamber 38 and preferably includes several helically shaped blades 46projecting from a shaft 48 which extends into the agitator chamber 38.The shaft 48 is rotationally driven by a motor and transmission unit 50mounted externally on a wall 52 of the agitator chamber 38. The motorand transmission unit 50 also has a junction box 54 mounted to the unit50.

Positioned below the agitator chamber 38 is an inlet section 40 of theauger assembly 18, with the section 40 for introduction of the pellets10 to a screw 42 mounted generally horizontally within the augerassembly 18. The screw 42 is rotationally driven by a motor andtransmission unit 56 mounted externally on the wall 52 below the motorand transmission unit 50 for the agitator 44. The rotational speeds ofthe motor and transmission units 50, 56 are preferably independentlyadjustable. The rotation of the blades 46 of the agitator 44 moves thepellets 10 toward the center of the agitator chamber 38 so that, as thepellets 10 are loaded into the inlet section 36, the blades 46 agitatethe pellets 10 and break up any agglomeration or blocking of the pellets10 prior to introduction of the pellets 10 into the inlet section 40.The screw 42 is rotationally mounted within the auger assembly 18 fortransfer of the pellets 10 from the particle feeder 16 to an outlet 58of the auger assembly 18.

In one aspect of the invention, the particle feeder/auger assemblysystem 16, 18 advantageously is a Thayer PF-5 volumetric feeder, modelnumber PF-5-6-4.8, available from Thayer Scale—Hyer Industries, Inc.,Pembroke, Mass.

This type of volumetric feeder is described in Andrews U.S. Pat. No.5,333,762, the entire disclosure of which is incorporated herein in itsentirety by reference. The feeder frame of the feeder section 36 isformed of 304 stainless steel, mill finish (2B), with the feeder sectionmaterial which comes in contact with the pellets 10 being formed ofelectropolished 304 stainless steel. The feeder section 36 isconstructed so as to meet Class I, Group C and D explosion proofingrequirements. In order to meet these explosion proofing requirements,the motor 50 is a high-efficiency, inverter-duty, 406 volt, 3 phase,constant-speed AC motor from the Reliance Electric Company. The screwlength from the center line 35 of the particle feeder inlet port 34 tothe center line 59 of the outlet 58 is 72 inches; if desired, anextension (not shown) may be provided, thereby allowing for a screwlength from center line 35 to center line 59 of 84 inches.

The screw 42 includes a center shaft 60 and closed, helical flights 62of uniform pitch P and outer diameter D_(s). The pitch P of the screw 42is six inches and the diameter D_(s) of the screw 42 is 6 inches. Therotation of the screw 42 within the chamber or barrel 32 sweeps crests64 of the helical flights 62 across an interior cylindrical surface 66of the barrel 32. The inside diameter D_(c) (or D_(B)) of the chamber orbarrel 32 is approximately 6 inches so as to provide for a minimumallowable clearance between the crests 64 of the screw 42 and theinterior surface 66 of the barrel 32 during rotation of the screw 42.The motor 56 is a model number 1305, 2.0 horsepower, 460 volt, 3 phase,60 Hz, variable frequency, AC motor from the Allen Bradley Company. Theparticle feeder/auger assembly system 16, 18 further includes anextension for an Allen Bradley Human Interface Module (HIM). The HIM ismounted on the front of the drive enclosure, and includes an analogspeed potentiometer having a feed-rate set point ranging from 4 to 20milliamperes.

The particle/feeder auger assembly system 16, 18, as shown in FIGS. 3and 4, includes at least one, preferably two, and more preferably four,spray head(s) 30 mounted to the barrel 32 of the auger assembly 18.Barrel 32 is drilled at specific locations for receiving the sprayhead(s) 30, and a mounting and locking member is supplied as describedbelow to facilitate flush mounting at the interior cylindrical surface66 of the barrel 32. Each spray head 30 is operatively coupled to asupply source 22 of the aqueous coating composition 12, and to an airsource 68 which assists in atomizing the composition 12, so that thecomposition 12 is delivered through the spray head(s) 30 into the barrel32 in an atomized state.

When more than one spray head 30 is used, an important feature of theinvention is the spacing of the multiple spray heads 30 relative to eachother and relative to the pitch P of the screw 42. For example, whenmultiple spray heads 30 are oriented in a straight line parallel withthe longitudinal axis of the barrel, as shown in FIG. 3, a first sprayhead 30 a is spaced 8 inches from a mouth 70 of the barrel 32 asidentified by a distance L in FIG. 3. Likewise, a second spray head 30 bis spaced 16 inches from the mouth 70 of the barrel 32 as identified byL₁. The third spray head 30 c is spaced 23 inches, as identified by L₂,from the mouth 70 of the barrel 32 and, finally, the fourth spray head30 d is spaced 30 inches from the mouth 70, as identified by L₃. Thespacing of the multiple spray heads 30 relative to each other is suchthat the spacing between any two of the spray heads 30 is not anintegral multiple of the six-inch pitch P of the screw. Additionally,the first spray head 30 a is spaced a distance S₁, of 8 inches from thesecond spray head 30 b. Likewise, the first spray head 30 a is spaced adistance S₂ of 15 inches from the third spray head 30 c and a distanceS₃ of 22 inches from the fourth spray head 30 d. As such, each of thespacings S₁, S₂, and S₃ are non-integral multiples of 6 inches, thepitch P of the screw 42. A comparison of the spacings of the other sprayheads 30 relative to each other reveals that these spacings also arenon-integral multiples of the pitch P. As one of ordinary skill readilywill appreciate from this description, although the spacing of the sprayheads 30 relative to each other is greater than the pitch P, the spacingbetween spray heads may be less than the pitch P, or may be a mixture ofspacings, some of which are greater than, and some of which are lessthan, the pitch, while still providing non-integral multiples of thepitch P.

As can be further appreciated by one of ordinary skill from the abovedescription, the particle feeder/auger assembly system 16, 18 performsthe function of conveying and mixing substrate particles 10 while anaqueous coating composition 12 is being uniformly applied via one ormore spray heads 30. Preferably, the substrate particle feed rate,aqueous coating composition feed rate, spray pressure, chamber (barrel)length, and screw rotation rate are adjusted to provide for a uniformlycoated particle at the outlet of the chamber (barrel). It can be furtherappreciated though not preferred, that the thickness of the crests ofthe screw and the spray head orifice spray pattern can be adjusted sothat the crests may pass through the spray pattern of at least one sprayhead while still providing application of aqueous coating composition tothe substrate particles. Thus, the spray head spacing relative to theposition of the crests of the screw is not a limitation on the scope ofthe invention. It can be still further appreciated that the screw mayhave a variable pitch.

One of the many benefits of the multiple spray head spacing aspect ofthe invention is that, although each spray head nozzle orifice (orificeto be discussed below) may become temporarily masked by a flight crest64 as the screw 42 rotates, at least one of the spray nozzle orificesalways will be free of a flight 64. Therefore, the flow of the coatingcomposition 12 through the spray heads 30 into the barrel 32 is nevercompletely interrupted. It should be understood by one of ordinary skillin the art that the particular numerical spacings of the spray heads 30and dimensions of the screw 42 of the particle feeder/auger assemblysystem 16, 18 of FIG. 3. are not limitations on the scope of theinvention.

Referring to FIG. 4, three spray heads 30 according to one version ofthe invention are shown, in which one crest 64 temporarily is positionedso as to at least partially block the spray head 30 c; however, due tothe spacing of the spray heads 30 relative to each other, as well as thepitch P of the screw 42, the other two spray heads 30 a, 30 b each areunobstructed, thereby enabling the composition 12 to flow through thesespray heads 30 a, 30 b onto the pellets 10 within the barrel 32 withoutany temporary masking by the flight crests 64. Each spray head 30includes an air-atomizing nozzle and a wall-mounting adapter 72, both ofwhich are commercially available from the Spraying System Company ofWheaton, Ill. as Model 4JCO and 3376 with ¾ inch NPT (M) connection,respectively. The spray head 30 includes a clean-out needle assembly 74mounted to a nozzle body 76, an air conduit 68 a and port 78 a fordelivering air to the nozzle body 76, from the air source 68 and anaqueous coating composition conduit 22 a and port 78 b for deliveringaqueous coating composition 12 to the nozzle body 76 from thecomposition source 22. An air cap 80 is concentrically seated onto afluid cap 82 which is mounted to the nozzle body 76 opposite from theneedle assembly 74. The air cap 80 and fluid cap 82 are directed towarda threaded opening 84 in the barrel. The fluid cap 82 includes a spraydischarge nozzle having an orifice 87 and being seated within thesurrounding air cap 80.

Preferably, each spray head 30 is mounted to the barrel 32 such that thedischarge nozzle orifice 87 is substantially flush with the interiorsurface 66 of the barrel 32, thereby providing for minimal clearancebetween the orifice 87 and a corresponding flight crest 64 of the screw42. This feature minimizes and even avoids formation of solids build-upinside the auger assembly while still permitting rotation of the screw42. As such, auger assembly 18 down-time, repair and cleaning isminimized, and the particles ejected from the nozzle 86 are moreuniformly coated onto the pellets 10 with the desired concentration.

A retaining ring 88 is threadably mounted onto a forward portion of thefluid cap 82 and securely retains the air cap 80 therein. In addition,the spray head 30 is securely mounted to the barrel 32 by the wallmounting adapter 72. The wall mounting adapter 72 provides for theaccurate positioning of the nozzle 86 relative to the interior surface66 of the barrel 32 and the crests 64 of the screw 42 as previouslydescribed. The wall mounting adapter 72 is generally in the form of aferrule having external threads 90 for mating with the threaded opening84 in the barrel 32. The wall mounting adapter 72 also includes internalthreads 92 for coupling to external threads 94 on the retaining ring 88of the spray head 30 and thereby securely mounting the spray head 30 tothe side wall of the barrel 32.

As the pellets 10 are advanced within the barrel 32 of the augerassembly 18, the aqueous coating composition 12 is sprayed through thespray heads 30, thereby coating the pellets 10. Additionally, the sprayheads 30 are advantageously spaced relative to one another and locatedrelative to the pitch P of the screw 42 within the auger assembly 18 toavoid completely blocking or interrupting the flow of aqueous coatingcomposition 22 onto the pellets 10 at any particular instance in timeduring rotation of the screw 42. After the pellets 10 pass the sprayheads 30, the rotation of the screw 42 continues to advance the coatedpellets toward the outlet 58 of the auger assembly 18 for subsequentparticle drying and cooling in a fluidized bed or the like.

As shown and described herein, the spray head(s) 30 is/are mounted ontothe barrel 32 of the auger assembly 18. Nevertheless, it should beappreciated that the pellets 10 may be coated with the composition 22 atother stages or locations within the scope of this invention. Forexample, spray head(s) 30 may be mounted on other parts of the augerassembly 18 such as the outlet 58 or other components such as theparticle feeder 16, the fluidized bed 24 or other pellethandling/processing components.

The fluidized bed preferably is constructed so that the wallconstruction along the edges of the horizontal portion of the bed isradiused, which facilitates motion of the coated pellets up the radiusedvertical wall portion of the bed followed by collapse onto the particlesin the horizontal portion of the bed. This motion has been foundbeneficial in efficiently drying the coated pellets. Although anysuitable fluidized bed may be used, particularly beneficial results maybe achieved using the Model FBP-1405 Fluid Bed Processor withSelf-Contained Vibratory Motor from Carman Industries, Inc.,Jeffersonville, Ind. Altenatively, a vibrating-drum style of fluidizedbed as available form General Kinematics of Chicago, Ill. may be used toadvantage.

Operating Examples

The following detailed operating examples illustrate the teachings ofthe invention in its most preferred form. The principles of thisinvention, the operating parameters and other obvious modificationsthereof, will be further understood in view of the following detailedexamples.

General Methods

Several of the illustrative examples provided below includestick-temperature values for the resulting coated particles whichdemonstrate the improvement in anti-block properties of the coatedparticles. The term “stick temperature” refers to the maximumtemperature at which the coated particles still are free-flowing, asdetermined using a stick-temperature test method developed and practicedby Equistar Chemicals, LP.

Stick temperature is determined using a stick temperature apparatusavailable from MBS Associates, Inc. of Cincinnati, Ohio under theproduct code number ONEQS002. The apparatus includes a control unitoperatively connected to a sample tube, with the tube being mounted on arotatable axis, thereby allowing the tube to be oriented in an uprightposition as well as in a dumping position. The control unit includes atemperature controller, an air inlet and an air outlet, timers toregulate air cycles, a heater for use in heating the sample tube, an airpressure regulator, and a flow meter to regulate the flow of air throughthe control unit and into the tube. In addition, the tube itselfincludes a removable, weighted top. The sample tube is formed ofstainless steel and has an inner diameter of 1⅝ inches and a height of12 inches. The weighted top assists in applying pressure to theparticular polymer contained within the sample tube, and is cylindricalin shape, having one larger diameter cylinder connected directly andcoaxially to a smaller diameter cylinder, with the smaller diameterportion being sized so as to be positioned part way into the sample tubethrough an opening in the top of the sample tube. In more detail, theweighted top is 8¼ inches tall, with the larger diameter portion havinga height of 5 inches, the smaller diameter portion having a height of 5inches, and the smaller diameter portion having a height of 3¼ inches.In addition, the diameter of the larger diameter portion is 3½ inches,whereas the diameter of the smaller diameter portion is 1½ inches.

Prior to running the stick test, the control unit is plugged into a 110volt outlet, and the air in-port is connected to a supply of ambientair. The control unit's air regulator then is turned on to the set level(i.e., 20-22 psig), and the unit's flow meter is set to a level of 33liters per minute. At this point, the unit's control panel is turned onand the temperature controller is set to the desired point, a pointwhich varies as a function of the particular material being tested. Inthe vast majority of testing situations, this desired starting point is50° C. Next, the weighted top is positioned in place on the sample tube,the heater switch is turned on to power the heater, and the timers areset for 900 seconds of heated air, 300 seconds of cool air, and 60seconds of dumping sequence. The weighted top then is removed from thestainless steel tube, and the tube is lined with a 6 inch by 11 inchsheet of polyethylene terephthalate (PET) film available from PilcherHamilton under the product name Phanex IHc Polyester Film.

At this point, 180 grams of the particular material to be tested isweighed out. When the temperature of the sample tube is stabilized atthe desired set point, the material is charged, (i.e., loaded) into, thesample tube. The weighted top is once again positioned on the top of thesample tube, and the test is begun by turning on the timer switch on thecontrol panel. After both the heating and cooling timers have reachedtheir set points, the control unit sounds an alarm. This alarm may besilenced by pressing the alarm button on the control panel of the unit.Next the weighted top is removed from the sample tube. The air line,which has been connected via a quick-connect coupling to the bottom ofthe tube, is disconnected from the tube, the safety pin is removed fromthe lever which is used to turn the sample tube in to a dumpingposition, and the tube is inverted for 60 seconds, with the materialexiting the tube being captured in a conventional weighing cup forsubsequent measurement.

After the 60 second dumping period, the control unit will sound anotheralarm which is silenced by pressing the alarm button on the controlpanel of the unit. At this 60-second point, the opening of the tube iscovered to prevent further discharge from the tube, and the tube isreturned to its original, upright position. In addition, the alarm isreset by pressing the alarm button. Next, the pellets which werecaptured in the weighing cup are weighed, and the percentage of thematerial recovered is calculated using the formula [weight out/weightin] ×100. At this point, any pellets remaining in the sample tube areremoved using a clean-out apparatus. This particular apparatus is 18inches long, 1⅜ inches wide, and 0.098 inch thick. The air line is thenreconnected to the sample tube, and the heat cycle is reset by turningoff the timer switch on the control unit.

If more than 90.0% of the pellets is recovered, the test is repeateduntil less than 90.0% of the pellets is recovered in the 60 second timeperiod. In each repeat test, the sample tube is charged with new,identical material, and the temperature of the sample tube is increasedfrom the temperature of the immediately preceding test by 2° C.

If less than 90.0% of the pellets is recovered, the test is repeatedwith a new sample until greater than 90.0% is recovered in the oneminute time period. In performing this retesting, the temperature of thesample tube is decreased by 2° C. from the temperature used for theimmediately preceding test.

The official “stick temperature” is the highest temperature, in degreesCelsius, at which 90.0% or greater of the sample material is recovered.

The coating particles applied to the substrate particles tend to remainin place even after being subjected to vibratory and abrasive forces asduring storage and transport. This tendency of the coating particles toremain on the surface of the substrate particles is evaluated by amethanol wash test, developed and practiced by Equistar Chemicals, LP.The methanol wash test is performed by weighing 100 grams, to thenearest 0.1 gram, of the particular polymeric coated particles to betested, and transferring the particles to a 500 milliliter Erlenmeyerflask. 50 milliliters of methanol is added to the flask, the flask isseated with a stopper, and shaken vigorously for 15 to 20 seconds todisperse the coated particles in the methanol.

Next, the suspension is decanted through a porcelain filter funnelcontaining a fiber glass filter which has previously been dried at 100degrees Celsius and weighed to the nearest 0.1 milligram. The 50milliliter wash step and the decanting step are continued until themethanol in the Erlenmeyer flask is observed by the naked eye to be freeof suspended polymeric coating particles. At this point, the porcelainfilter funnel containing the fiberglass filter and entrapped coatingparticles is rinsed with methanol and dried for one hour at 65° C. Then,the crucible is cooled in a desiccator and weighed to the nearest 0.1milligram to determine the total weight of these polymeric coatingmaterial removed from the coated particle. The “weight percent removed”is calculated by the following equation: [weight of polymeric coatingparticles recovered (grams/initial weight of coated polymeric particlesample) grams] ×100.

Except as otherwise noted, all pellets prepared for use in the examplesbelow were formulated with 500-750 ppm antioxidant, typically BHT(butylated hydroxytoluene) oroctadecyl-3,5-di-tert-butyl-4-hydroxyhydrocinnamate such as Irganox®1076 from Ciba Geigy, introduced into the extruder barrel prior topelletization as a 25% solids slution by weight in mineral spirits.

EXAMPLE 1 Formation of Coated Pellets Using the Auger System

In providing the coated pellets, 1 lb of deionized water was added to200 lb of ethylene-vinyl acetate copolymer (EVA) pellets as apre-wetting agent used to prepare the surface of the pellets prior tothe application of the coating. The EVA pellets used had a melt index of410 g/10 min, a 28% vinyl acetate content by weight, and are availablefrom Equistar Chemicals, LP, Cincinnati, Ohio. 200 lb of the EVA pelletswere fed through the auger system, during which time the pellets werecoated with an aqueous coating composition of an EVA powder dispersed inan emulsion. The EVA powder used was Microthene® FE 532-00 (9MI, 9% VA)available from Equistar Chemicals, LP, Cincinnati, Ohio, and theemulsion in which the Microthene EVA powder was dispersed was anAquastab® emulsion from the Eastman Chemical Company, Kingsport, Tenn.,as described in conjunction with Table 1 above.

The aqueous coating composition was delivered through the flush-mountedspray nozzles of the auger system onto the EVA pellets moving throughthe auger system. The auger system was a lab scale unit which utilizedtwo spray heads spaced at a non-integral multiple of the pitch of thescrew. The barrel diameter was 4 inches. The auger system included anagitator bar in the hopper.

The coating composition contained 40 weight % Microthene® FE 532-00, hada viscosity of 300 centipoise (cP), and was applied with an air pressureof 40 pounds per square inch gauge (psig) in order to spray the coatingcomposition in an atomized form. In addition, the aqueous coatingcomposition was applied at a 1% concentration, resulting in a coatingparticle concentration of about 4000 parts per million (ppm), ie., 0.40weight % basis 100% solids to 200 lb of pellets. The coated pelletssubsequently were passed to a fluidized drying bed, where the coatedpellets were dried on a Process Control Corporation static fluid bedusing room temperature air at an airflow rate in the range of about 300to 500 standard cubic feet per minute (scfm). The airflow was generatedby a trim blower from North American Manufacturing. The collected samplethen was analyzed for stick temperature and pellet performance . Whereasthe initial EVA pellets had a stick temperature of 30° C., the coatedEVA pellets now had a stick temperature of 52° C. The increase in sticktemperature was 22° C., resulting in an improvement of 73%. Even afterwashing the coated pellets 6 times using the methanol wash testdescribed above, the coated pellets still had a stick temperature ofabout 36° C.

EXAMPLE 2 Drying the Coated Pellets

After the pellets coated in Example 1 had been formed but not yet dried,they were temporarily stored 5 days in a Gaylord container andsubsequently dried in a vibrating, fluidized bed. The material was runthrough the bed at a rate of about 550 pounds per hour, with the airtemperature being about 120° F. The dried, coated particles then werestored in an Equistar Chemicals, LP. facility in a clean Gaylordcontainer for subsequent experimentation.

EXAMPLE 3 Testing the Anti-Block Characteristics

Approximately four months after storing the dried, coated pellets ofExample 2 in the Gaylord container, a sample of these coated pellets wasremoved from the Gaylord container using a “sample thief”. The samplethief is a tube which is about 3.5 feet long and which has severalopenings through which the coated pellets may enter into the interior ofthe tube. In addition, the sample thief is capable of holding about 100grams of coated pellet material. In performing this example, the samplethief was inserted into the Gaylord container five different times atfive different locations, with sample collected for stick temperaturetesting. No problems were encountered with use of the sample thief, evenwhen the sample thief was inserted to the bottom of the Gaylordcontainer.

EXAMPLE 4 Stick Temperature Test of the Coated Pellets of Example 3

A stick temperature test was performed on a sample of coated pelletswhich had been collected in Example 3. After four months storage, thesecoated pellets exhibited a stick temperature of 52° C. The increase instick temperature was 22° C., resulting in an improvement of 73%.

GENERAL METHOD FOR EXAMPLES 5-7 Formation of EVA Pellets Coated with aCoating Composition

Ultrathene® UE 653-04 EVA pellets with a 410 melt index (MI) and 28%vinyl acetate content (VA) and an uncoated stick temperature ofapproximately 30° C. were coated with the aqueous coating compositiondescribed in detail in Example 1. In each of Examples 5-7, theappropriate amount of the aqueous coating composition described inExample 1 was diluted with a small amount of deionized water in order toform a diluted aqueous coating composition which would provide thedesired parts per million (ppm) of polymeric coating particles remainingon the pellets after the coated pellets had been dried. In order to coatthe pellets, 1000 grams of Ultrathene® UE 653-04 pellets were placedinto a plastic bag, the appropriate amount of diluted aqueous coatingcomposition was poured into the plastic bag, and the combined contentsof the bag were shaken for about five minutes to enable the aqueouscoating composition to contact all of the pellets. The contents of theplastic bag then were dried in an open pan at room temperature andatmospheric pressure.

EXAMPLE 5 EVA Pellets Coated with 3000 ppm EVA Powder

7.5 grams of the aqueous coating composition were diluted with a smallquantity of deionized water in order to form a diluted aqueous coatingcomposition which would achieve a coating level of 3000 ppm on the EVApellets. The resulting coated pellets had a stick temperature of 48° C.The increase in stick temperature was 18° C., resulting in animprovement of 60%.

EXAMPLE 6 EVA Pellets Coated with 4000 ppm EVA Powder

10.0 grams of the aqueous coating composition were diluted with a smallquantity of deionized water in order to form a diluted aqueous coatingcomposition which would achieve a level of 4000 ppm of polymeric coatingparticles on the resulting coated pellets. The resulting coated pelletshad a stick temperature of 46° C. The increase in stick temperature was16° C., resulting in an improvement of 53%.

EXAMPLE 7 EVA Pellets Coated with 5000 ppm EVA Powder

12.5 grams of the aqueous coating composition were diluted with a smallquantity of deionized water in order to form a diluted aqueous coatingcomposition which would achieve a polymeric coating particle level of5000 ppm. The resulting coated pellets had a stick temperature of 50° C.The increase in stick temperature was 20°, resulting in an improvementof 67%.

EXAMPLE 8 Results of Methanol Wash Test

A sample of coated pellets formed as described in Example 1 weresubjected to the methanol wash test described in the General Methodssection. The tenacity of the polymeric coating particles for thepolymeric substrate pellets was such that the methanol wash had to beperformed 7 times before the methanol was observed to be free ofsuspended polymeric coating particles.

Thus there is disclosed a coated particle, method of making the coatedparticle, and equipment for facilitating manufacture of the coatedparticle. In addition to the advantages of the coated particlepreviously described, the coating imparts an enhanced slipperiness tothe coated particles which permits denser packing of polymeric particleswhich have received the coating. Thus, a greater weight of coatedparticles may be stored in a fixed volume compared to particles whichdid not receive the coating.

What is claimed is:
 1. A system for making coated polymeric particlehaving improved anti-block characteristics, comprising: a chamber havingan interior sidewall, an interior space, and an inlet spaced from anoutlet; a rotatable screw, at least a portion of the screw beingpositioned within the chamber interior space, the screw having a shaftand at least one helical flight extending radially outward from theshaft, the helical flight having a crest and a pitch; a rotational driveoperatively coupled to the screw; a first spray head in fluidcommunication with the interior space of the chamber, the first sprayhead having a discharge nozzle including an orifice, with the orificebeing substantially flush with the interior sidewall of the chamber; anda second spray head in fluid communication with the interior space ofthe chamber.
 2. The system of claim 1 wherein the second spray head influid communication with the interior space of the chamber and havinghas a discharge nozzle including an orifice, the second discharge nozzleorifice being positioned along the interior sidewall such that when thecrest of the helical flight passes across the first discharge nozzleorifice, the crest does not pass across the second discharge nozzleorifice.
 3. The system of claim 2 wherein the first and second sprayheads are oriented in a straight line parallel to the longitudinal axisof the chamber.
 4. The system of claim 1 wherein the interior sidewallis substantially circular in cross-section.
 5. The system of claim 4wherein the interior sidewall has an inside diameter (D_(c)) and thescrew helical flight has an outside diameter (D_(s)), the outsidediameter being substantially the same as the inside diameter.
 6. Thesystem of claim 1 further comprising an agitator in fluid communicationwith the chamber.
 7. The system of claim 1 further including a coatedpolymeric pellet drying apparatus in fluid communication with thechamber.
 8. The system of claim 1 wherein the second spray head has adischarge nozzle including an orifice, with the orifice beingsubstantially flush with the interior sidewall of the chamber.